Can't use image as world texture
If you are trying to use the image as a sky/panorama background, you probably want the Environment texture node instead of the Image texture node.

The environment node will let you use panorama projection types (Mirror Ball and Equirectangular) This is the only difference between the two nodes, see What are the differences between an environment node and a image node?.



However, if you really want to use the Image node for the sky/background (e.g. to have an image stuck exactly behind the camera, always in the same orientation), you will need to adjust the coordinates with a Mapping node:



Note that to edit the world nodes, you must ensure the little globe icon in the header is selected.